In motor control applications requiring forward and reverse motor rotation, a bridge circuit is generally used to supply current to the motor armature or field windings. In its basic configuration, the bridge circuit resembles an H, each leg of the bridge comprising a power switching device such as a power transistor. The motor winding is connected across one pair of opposing bridge terminals and a DC power source is connected across the other pair of opposing bridge terminals. Current of positive or negative polarity may be supplied to the motor to produce forward or reverse motor rotation by biasing one or the other pair of diagonally opposed power transistors conductive. Freewheeling diodes are connected across each leg of the bridge to protect the respective power transistor at turn-OFF by providing a shunt circuit path for inductive motor current.
Driver circuits are used to control the conduction of the bridge power transistors, and may operate the bridge in either the one quadrant control mode or the two quadrant control mode. In the one quadrant control mode, one of the power transistors is maintained conductive and the diagonally opposed power transistor is modulated conductive and nonconductive to effectively connect and disconnect the motor winding and the power source. Inductive current stored in the motor winding when the modulated transistor is biased nonconductive is circulated through the bridge via the conducting transistor and a freewheeling diode. Thus, the term "one quadrant control" indicates that the power source supplies current in one direction only. In two quadrant control, diagonally opposed power transistors are biased conductive and nonconductive in unison and inductive current stored in the motor winding when the power transistors are biased nonconductive is circulated through the power source via two freewheeling diodes. Thus, the term "two quadrant control" indicates that the power source supplies/accepts current in two directions.
In operating the motor, power losses are inherently incurred in both the driver circuits and the power transistors. One quadrant control results in relatively low switching losses in the power transistors, but relatively high driver losses since one driver circuit is provided for each power transistor. On the other hand, two quadrant control results in relatively high switching losses in the power transistors, but relatively low driver losses since a single driver circuit may be used to control two diagonally opposed power transistors.